General-Purpose
Computer and Communication Interface
The five GPCs
in the orbiter are loaded with software (to perform different functions)
during different phases of the mission. During ascent and entry,
four of the GPCs are loaded with primary avionics software system
to provide guidance, navigation and control. One computer is loaded
with backup flight system software and can take control of the orbiter
if the PASS GN&C; computers malfunction. The BFS GPC also contains
all software necessary to return the orbiter to Earth, if required.
Normal communication commands go through the BFS during ascent and
entry phases of the mission. During the on-orbit phase of the mission,
one computer is loaded with systems management software to support
the communication commands. These two GPCs process antenna-pointing
control commands for the S-band and Ku-band antenna systems. They
also provide a communication command path from the ground to the
required subsystem as well as a telemetry downlist path to the PCMMU
to be interleaved with other data for transmission to the ground.
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